1. Field of the Invention
The present invention relates to a capacitor microphone, and more particularly relates to a shield structure of a capacitor microphone including a built-in battery.
2. Description of the Related Art
Generally, a capacitor microphone includes an impedance transformer since a microphone unit suffers from very high impedance. A field effect transistor (FET) is usually used as the impedance transformer, and a vacuum tube is utilized in a rare case.
The capacitor microphone outputs voice signals via a shielded and balanced cable connected thereto. A 3-pin connector (specified in EIJI, RC-5236, “Latch-locked type Round Connector”) is used to connect the foregoing components. (Refer to Japanese Patent Laid-Open Publication No. Hei 11-341,583, for example.) With the 3-pin connector, No. 1 pin is for grounding, and No. 2 and No. 3 pins are used as a hot side and a cold side of a signal wire, respectively. XLRM type 3-pin connectors manufactured by Canon Kabushiki Kaisha are available on the market.
Usually, the capacitor microphone is connected to a phantom power source using the balanced and shielded cable, and receives the electric power from the phantom power source. However, a super-directional gun-shaped microphone which is frequently used outdoors includes a size AA battery, and is operated by the electric power from the battery. An example of a capacitor microphone including a built-in battery power source will be described with reference to FIG. 4 to FIG. 6 of the accompanying drawings.
Referring to FIG. 4 to FIG. 6, a cylindrical coupler 11 is provided with a female screw 11a and two male screws 11b and 11c in a rear end. The female screw 11a is at the right end of the coupler 11, while the male screws 11b and 11c are positioned outside the female screw 11a. The male screws 11b and 11c are adjacent to each other. The coupler 11 is inserted form its front end into a microphone unit housing (not shown) with the male screw 11c fitted into a female screw of the microphone unit housing. The microphone unit housing houses a capacitor microphone. In order to block electromagnetic waves, the microphone unit housing and the coupler 11 are made of conductive materials such as an aluminum or a brass alloy.
At the rear end of the coupler 11, a male screw 20a on an outer front surface of a battery holder 20 is fitted into the female screw 11a of the coupler 11. The battery holder 20 has a battery recess 21 in the shape of a ship bottom in order to house a size AA battery, for instance. A positive contact and a negative contact (not shown) are provided at the front and rear ends of the battery recess 21. Necessary wirings are connected to the positive and negative contacts. The battery holder 20 except for the positive and negative contacts is made of a resin material.
A male screw 20b is provided around a rear outer end of the battery holder 20, and is fitted into a female screw 30a of a cylindrical conductive connector sleeve 30, so that the battery holder 20 and the connector sleeve 30 are coupled. The female screw 30a is provided in the front end of the connector sleeve 30. An output connector 40 is fitted into the connector sleeve 30. A setscrew radially passing through the connector sleeve 30 is fitted into the output connector 40, so that the output connector 40 is fixedly attached. A groove 31 extends on an inner peripheral surface of the connector sleeve 30. The output connector 40 is housed in the connector sleeve 30.
A 3-pin connector specified in EIJI RC 5236, “Round Latch type Connector for Acoustic Device” is used as the output connector 40. No. 1 pin, i.e., a grounding pin, is electrically connected to the connector sleeve 30 via a conductive screw (not shown). The output connector 40 is detachably connected to a plug at one end of the balanced and shielded cable (not shown). The plug is provided with a grounding sleeve at a position where the plug is fitted into the connector sleeve 30. The grounding sleeve is connected to a shielded wire of the balanced shielded cable. A locking claw is provided at a part of the grounding sleeve, and is engaged in the groove 31 of the connector sleeve 30 when the grounding sleeve is fitted into the connector sleeve 30. The locking claw is disengaged from the groove 31 by operating a knob. A mechanism for detaching the connector 40 from the balanced shielded cable is well-known, and is not shown.
As shown in FIG. 5 and FIG. 6, a cylindrical battery cover 50 is attached around the output connector 40 and the battery holder 20 by fitting a female screw 51 at the front end of the battery cover 50 into the male screw 11b of the coupler 11. The battery cover 50 is long enough to extend around the battery holder 20 and the output connector sleeve 30 when fitted into the coupler 11. The battery cover 50 can be disengaged from the coupler 11, and is moved backwards, so that the battery holder 20 will be detached. In this state, the battery may be exchanged with a fresh battery. Thereafter, the battery cover 50 is again attached and fitted into the coupler 11.
With the capacitor microphone operated by the built-in battery above, the battery holder 20 is adjacent to the output connector 40, and it is not easy to ground the battery cover 50. Therefore, it is very difficult to prevent noises caused by high frequency electromagnetic waves.
The inventor has proposed a capacitor microphone in which a conductive leaf spring comes into contact with a battery cover, and the battery cover is connected to a shield wire of a microphone cord, as disclosed in Japanese Patent Application 2004-209,981.
With the foregoing capacitor microphone, noises caused by electromagnetic waves can be reduced because the battery cover functions as a shield. However, since the leaf spring is in point contact with the battery cover, it is very difficult to block strong electromagnetic waves having high frequencies emitted from cellular phones which are currently very popular. Therefore, noises will be generated if the capacitor microphone is used at close range of cellular phones.